An SSD (Solid State Drive), which is a storage device that is provided with a nonvolatile semiconductor storage medium (hereafter referred to as a semiconductor storage device), is publicly known. Since a high performance of a read and a write of data for the SSD can be obtained, the SSD is being utilized for a storage system. However, a semiconductor storage device is costly as compared with a storage device such as an HDD, and it is not suitable that all data that are stored in a storage system is stored into a semiconductor storage device from a point of view of a cost. On the other hand, it is known that a read write frequency is provided with a locality for data of a storage system in general. Consequently, in the case in which only data of a high read write frequency is stored into a semiconductor storage device and data of a low read write frequency is stored into a storage device such as an HDD (Hard Disk Drive), an average high performance of a storage system can be expected at a low cost.
The SSD described above is an electrically rewritable semiconductor storage device. The SSD is provided with a plurality of semiconductor storage media and data is stored into each of the semiconductor storage medium in general.
A semiconductor storage medium is composed of a plurality of cells and maintains two states for one cell to the passage of time. More specifically, one of the two states of a cell is a bit 1 and the other of the two states of a cell is a bit 0. For a flash memory that is one of semiconductor storage media for instance, a state in which an electron has been implanted to an FG (Floating Gate) of a cell is a bit 0 and a state in which an electron has not been implanted to an FG of a cell is a bit 1. In the case in which this state is maintained even if time elapses, data is held with propriety.
In recent years, a storage density of a semiconductor storage medium has been improved to lower a bit cost (a cost for holding data per bit). An improvement of a storage density is carried out by two methods for instance. One method is a miniaturization of a manufacturing process of a semiconductor storage medium. The other method is technique that is called a multiple value method, in which data of a plurality of bits is stored to one cell by classifying an implanted amount of an electron for one cell into a plurality of stages. By these techniques, a bit cost of a semiconductor storage medium is being lowered. However, a reliability related to a data hold is sacrificed by the reduction of a bit cost.
A deterioration of reliability due to a miniaturization and a multiple value method is caused by a reduction of a design margin. Since a cell is damaged by slow degrees due to a repetition of a write and an update of storage data as a characteristic of a semiconductor storage medium, a specific state cannot be held for a long period of time and is changed. In the case in which a design margin to a permissible amount of the change, the storage data can be held even if a state is changed. However, in the case in which a design margin is less, the storage data is changed even if a state change is minute. For instance for a flash memory, it is known that a hole trap is generated in an oxidation insulation film for keeping an electron that has been stored to an FG and an insulation property is degraded. In the case in which a design margin is reduced by the miniaturization and the multiple value method, an influence of an electron amount change in an FG due to a degradation of an insulation property is easily applied relatively, and it is difficult to hold data on a long-term basis. Consequently, a life of a semiconductor storage medium is to a damage state in which a reliability can be maintained, and the total update write data amount (hereafter referred to as write data amount) to each region of a semiconductor storage medium must be limited.
Due to a limitation of a write data amount of a semiconductor storage medium, in recent years, a product in which a write data amount is limited has been published for an SSD in which an element is a storage device (see Non Patent Literature 1 for instance). For an SSD in which a write data amount is limited, even in the case in which an indemnity period is 5 years, at a time when a write data amount to an SSD exceeds a permissible value, a guaranty of a reliability related to a storage hold of data cannot be held and a life of an apparatus is ended. Consequently, a life of a storage system that is provided with an SSD is decided in accordance with the write data amount. In the case in which a storage system is tried to be operated continuously, it is necessary that an SSD that outlives its usefulness is exchanged. In this case, since a life of an apparatus is shortened according to a write data amount, an operation cost of a storage system for a certain period is varied by a write data amount. In the case in which a write data amount of a system is larger, an SSD is degraded and an operation period of the total storage system is shortened, whereby a cost is increased.
On the other hand as described above, for an HDD that is one of storage devices of a low performance and a low bit cost, a limitation of a write data amount is less than that of an SSD. Consequently, for an HDD, a cost for updating write data (hereafter referred to as a write cost) as well as a bit cost is lower than that of an SSD. For a storage system that is composed of a plurality of storage devices including an HDD and an SSD, it is necessary that not only a bit cost is considered but also a write cost is considered in order to select a storage destination of data.
Moreover, in the case in which an operation period (a duration period) of a storage system has a priority over a performance (that is, a write cost is equal to or less than a certain value), it is thought that an operation period of a storage system is extended by setting that data of much write data amount is stored into an HDD.
Moreover, in the case in which a storage system is shared by a plurality of services (applications), it is necessary that the total amount of data that has been written to an SSD by each service (a write data amount) is considered to estimate an operation cost for every service in an accurate fashion. For instance, in the case in which a life of a storage system in which a life of 5 years is expected is 3 years due to an influence of a specific service in which a large amount of data has been written to an SSD among a plurality of services that share an SSD for a storage system, a user must recognize that an operation cost of a specific service in which a life is shortened is higher than that of other services.